David Baldacci, writer extraordinaire of airport thrillers, introduced his policeman/detective hero, Amos Decker, in 2015 and has pumped out one such every year since. The sixth, “Walk the Wire,” launches speedily, with Decker (gifted with perfect memory and synesthesia) travelling to North Dakota with a colleague, charged with investigating a gruesome murder in the snow. A seemingly routine, if intriguing, murder inquiry soon explodes into a fiendishly complex plot involving fracking (interestingly portrayed), a military base, and shadowy forces from Washington. Decker is an appealing, bluff, troubled character, and the author employs a well-etched cast of characters. The savage ice-bound environment of the Badlands is brought to life. Baldacci can write fluently with subtlety. All well and good, one might say, and I was looking forward to a pacy read to match the enjoyable, well-crafted Decker debut of “Memory Man,” but from the halfway point, “Walk the Wire” turns sour. Extravagant plot flourishes escalate, an assassin (star of another Baldacci series) enters the fray, and the action turns cartoonish. Plot twist after silly plot twist mars the capable execution. The finale will appeal to James Patterson fans but, in my opinion, lacks narrative depth. A disappointment.